The present invention relates to a pneumatic tire, more particularly to a bead structure suitable for heavy duty tires improved in the bead durability.
In recent years, reuse of heavy duty tires through retreading come to the front in view of resource saving and the reduction of carbon dioxide emissions. Thus, demand for heavy duty tires having a tire structure, especially a bead structure endurable over two life cycles have risen markedly.
In general, the heavy duty tires for trucks, busses and the like are, as shown in FIG. 5, provided with a bead portion (4) having an acute-angled bead toe (bt). Such acute-angled bead toe closely contacts with the bead seat (4Sa) of the wheel rim when the tire is mounted on a wheel rim, and provides a tight air sealing effect.
Conventionally, a hard rubber compound is used as the so called chafer rubber (23) extending from the axially outer surface of the bead portion to the bead toe through the bead bottom (bs). When the tire is mounted on the wheel rim or demounted from the wheel rim, for example as shown in FIG. 5 in dashed-dotted line, the bead toe (bt) is deformed largely in order to go over the rim flange (Rf). Thus, the bead toe (bt) is liable to be flawed and damaged by the rim flange.
In order to withstand such a large deformation, if the bead toe is formed from a relatively soft rubber compound in stead of the hard rubber compound, then the damage of the bead toe during mounting and demounting operations may be reduced. But, if such soft rubber is used beneath the bead core (5), the rubber is very liable to be crushed between the bead core (5) and the bead seat (4Sa) since a large compressive force is caused therebetween during use especially heavy duty use.
In either case, namely, if the flaws in the bead toe or crashing beneath the bead core is caused, the tire can not be reused.
A combination use of two compounds, namely, a soft rubber compound for the bead toe and a hard rubber compound for the bead bottom, is conceivable, but difficult because the bead toe and bead bottom are contiguous and, if the interface between such different compounds exists in the bead base portion, a separation failure more likely occurs.
Therefore, the present inventor studied and discovered that: in most cases where the relatively soft rubber chafer is crushed, the bead reinforcing layer (20) causes a cord angle variation, specifically, an increase in the cord angle with respect to the tire radial direction; and the smaller cord angle is more disadvantageous to the resistance to crushing; and that by increasing the cord angle up to a value between 30 to 60 degrees, a relatively low modulus range by which the flaws in the bead toe can be avoided, can encompass a modulus range within which the crushing can be also prevented.